gl_MaxDrawBuffers is still available in OpenGL ES 3.0.
It was just a matter of making it part of the common builtins.

Change-Id: Icd3ca9f4f50cba43c345974ef72603800155a573
Reviewed-on: https://swiftshader-review.googlesource.com/13389Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>